The invention relates to a method for producing hot-rolled hollow sections of steel. Furthermore, the invention relates to a hollow section produced by this method as well as to the use of such a hollow section. In particular involved are hollow sections which deviate from the circular shape such as for example hot-finished hollow sections according to EN 10210-2.
The hot-rolled hollow sections known as MSH®-sections can be produced from cold-finished longitudinally seam-welded pipe blanks as well as from hot-finished seamless pipe blanks having a substantially round starting cross section.
When using cold-finished longitudinally seam-welded pipe blanks, a round pipe is produced from a steel strip formed into an open seam tube, usually by means of HFI welding, and, after being heated to forming temperature, is hot-rolled in respective section rolling stands into a hollow section.
When using hot-finished seamless pipe blanks, the latter are either re-heated at the same heat or to rolling temperature and then formed with respective rolls to a hollow section having the required rectangular or square cross section.
Accordingly produced hollow sections are primarily used as construction pipes in addition to the classic field of use in steel frame superstructures increasingly also in industrial construction, sports facility construction, bridge construction and mechanical engineering, in machinery for structural engineering, special vehicle construction, commercial vehicle construction, agricultural machinery construction as well as in steel construction and building construction.
Increasing demands for environmentally friendly and more economical methods of construction have led to the development of hot-rolled hollow sections that, with regard to the required geometrical moment of inertia or section modulus, are lighter in weight or have a greater cross section, while having same nominal dimension (edge length×nominal wall thickness), and compared to hollow sections which for example are produced from bent sheet metal by cold-finishing, have significantly smaller edge radii or visible edges.
Because in practice, the edge radii of the hollow sections are uneven, EN 10210-2 does not specify the outer rounding radii but the lengths of the rounding regions, referred to in the following as visible edges.
The advantages of smaller visible edges in hollow sections are, on one hand, a greater geometrical moment of inertia and greater bending and torsion stiffnesses, and, on the other hand, a smaller welding joint is established at connections in the region of the section edge and thus a more appealing appearance, which is very important for exposed constructions.
Moreover, a wider support area for cross sections to be connected is realized so that the load-carrying capacity is increased. Moreover, a force can at least partially be introduced rectilinear into a section wall extending parallel to the force, when the visible edge is smaller than the wall thickness, a fact which is advantageous for static dimensioning.
Hot-finished hollow sections with a maximum permissible visible edge according to EN 10210-2 of ≦3.0×t (t=wall thickness) are, however, no longer adequate for all fields of application.
An example involves the field of application for rotating tower cranes with climbing capability for high-rise building construction. In these cranes, the tower height, and with this the crane height and hook height, are increased incrementally by inserting tower elements, and are thus adjusted to the progress of the building construction.
The tower elements are assembled from square or rectangular hollow sections, wherein the vertically extending corner sections of the tower element, the so called “corner posts”, are connected to one another by bolts arranged in the middle of the hollow section.
For adding tower elements, a guiding frame is used along the vertically extending corner sections of the tower element and has guide rollers which have to run very close to the section edge for reasons of space as a result of the bolted connection. When the visible edges are too large, the guide rollers would run too far in the middle of the hollow section so that the individual tower elements could no longer be connected by the bolts.
Further fields of application for hollow sections with smaller visible edges are, e.g., the bottom chords of outriggers of trolley-type rotating tower cranes to provide a wider support surface for the trolley rolls or generally sections which are subjected to bending stress, e.g. the crane track support system of steel for high loads known from DE 10 2007 031 142 and the support system of steel for roof construction known from DE 10 2006 010 951.
The maximum permissible value for the visible edge length C1 and C2 of ≦3.0×t for hot-finished hollow sections according to EN 10210-2, is clearly too high for the aforementioned fields of application. According to EN 10210, as a matter of principle, a value of 1.5×t is therefore used as a basis for calculation.
Heretofore, the use of a rolling process has not been successful to reduce the lengths of visible edges in hollow sections made from cold-finished welded or hot-finished seamlessly produced pipe blanks so as to render these hollow sections economically useful for the described fields of application. Therefore, a compromise was always sought between rolling capability of the hollow section, wear of the rolls, and the visible edge length; this however has not led to the desired success.
For that reason, these applications predominantly involve the use of, for example, hollow sections which are formed by welding two L-shaped legs to one another, and which at ≦1.0×t have very small visible edge lengths and thus are significantly below the standard for hot-finished hollow sections.
However, as a result of the welding seam, these hollow sections formed by welding individual sections to one another have the disadvantage of non-homogenous material properties, and as a result of the internal stress of the welding seam carry an increased risk of warping and their manufacture is elaborate.